1. Field of the Invention
The present invention relates to a data communication network supporting mobile devices, and particularly to an amplify and forward relay method that provides reliable data transmission in such a data communication network using relay stations.
2. Description of the Related Art
In wireless applications, users may not be able to support multiple antennas due to size, complexity, and power limitations. Cooperative diversity is a promising solution for such a condition. Among the known relaying schemes are the amplify-and-forward (AF) and the decode-and-forward (DF). In the AF scheme, the relay simply amplifies the source signal before forwarding it to the destination. In the DF scheme, some signal processing needs to be performed before the signal is forwarded. In systems where multiple relays are used, efficient relaying protocols, e.g., fixed, selection, and incremental relaying, have been proposed. In fixed relaying, a set of relays is used to forward the source signal to the destination. In selection relaying, a relay or a number of relays is selected to cooperate according the conditions of their channels. Finally, in incremental relaying, a relay or a set of relays is asked to cooperate based on a feedback signal from the destination if the direct link channel is under a certain threshold.
Other technologies include user cooperation in a coded cooperative system, where convolutional codes are used in a Rayleigh fading environment with path loss effect. Opportunistic relaying is a scheme in which the relay with the strongest end-to-end (e2e) signal-to-noise ratio (SNR) is selected to cooperate with the source. Hence, the channels of all relays need to be estimated each time in order to select the best relay among all relays. A study on centralized selection relaying has been presented in which all channel gains are assumed to be available at the destination node in order to select the best relay among all relays. Additionally, a known partial relay selection scheme for AF (amplify-and-forward) relay networks provides a relay selection algorithm where the relay with the SNR that is greater than a predetermined SNR threshold and is the maximum among other relays is chosen to be the best. In another selection relaying method, the protocol selects the second or even the Nth best relay with the highest end-to-end (e2e) SNR in the case when the first best relay is involved in some scheduling or load balancing schemes. Other schemes include the one where the relay with the best minimum of the two hop channels is selected as the best, and another one where the relay with the best value of a modified expression of the harmonic mean is selected to cooperate with the source.
In relay selection schemes for channel state information (CSI)-assisted dual-hop AF relay networks over Nakagami-m fading channels, the key idea is that the selection criterion is based on the channel magnitudes and not the channel SNRs, which is an attempt to reduce the system complexity. In incremental opportunistic relaying, the best relay is asked to cooperate if the direct link channel is below a predetermined SNR threshold. Additionally, energy-fair decentralized relay selection techniques in wireless sensor networks whose nodes are uniformly distributed according to a two-dimensional homogeneous Poisson process have been developed. These schemes take the network topological structure into consideration. Other researchers have proposed a relay selection scheme for half-duplex relays with buffers. This protocol guarantees that each time, the best first hop and second hops links are involved in the data transmission.
As can be seen, most of the aforementioned methods suffer from a heavy estimation load. As an example, in the best relay selection scheme, all channels of all relays need to be estimated each transmission time. On the other hand, in the partial relaying protocol, half this estimation load is required each time. This means more power consumption, low battery life, and high system complexity. In most wireless networks, once the minimum system requirement is achieved, no more operations that increase the system complexity need to be done. This finds its practicality in both sensor and ad-hoc networks.
Thus, an amplify and forward relay method solving the aforementioned problems is desired.